Canadian Patent 2,219,053 to Rowney (“'053”) teaches that wells for oil, gas and the like are typically drilled by a rig that includes a hollow drill string with a bit at its lower end. As the drill string is rotated, drilling fluids are pumped down through a channel in the drill string. The drilling fluids pass through the bit and return to the surface (as a slurry) on the outside of the drill string, carrying cuttings to the surface where the drilling fluids are recovered and recycled. Water may be used as drilling fluid in shallow drilling, but in deeper wells drilling “muds” of various compositions are used. Cuttings and other undesired solids are separated from recovered drilling fluids before re-use of the drilling fluids. A known but expensive method for mechanically separating solids from drilling fluid, which is employed in '053, is to pass the slurry through a centrifuge. Due to the large volume of drilling fluid used in a typical drilling operation, it is necessary to have either a very large centrifuge or several smaller centrifuges when processing in the manner taught by '053. The capital and operational costs of processing slurry using a centrifuge-based system are significant because large capacity units are expensive and consume large quantities of electrical power, which power is not always available at drilling sites. '053 also teaches baffles used in the traditional manner (to interrupt and slow the movement of slurry) installed in an elongate “settling tank” having a large open trough and auger extending substantially its entire length along the bottom of the tank; disadvantageously, the relatively large opening and the mixing action of the auger of '053 tend to allow access by substantial amounts of water from the tank to the solids outlet as well as agitating the solids and blending them with further excess water. The failure to prevent or reduce the amount of water reaching the centrifuges causes a relatively low efficiency due to excess water circulating through the centrifuges reducing the mass of solids being processed during a period of operation.
Known slurry processing technologies employ a variety of means to extend the path length between the tank inlet and solids outlet in order to permit separation by permitting solids to passively settle to the tank bottom. Various sloping surfaces are employed to resist the passively separated solids sticking to the inside of the tank. Disadvantageously, this passive distribution of solids permits substantial water to be drawn from the settling tank into the centrifuge where power is consumed in mechanically separating excess water from solids.
Other prior art relating to separating drilling fluid from slurry is based on flat bottom settling tanks having several chambers separated by baffles that extend to the bottom of tank. Openings are provided in the bottom or top of those baffles to limit flow between chambers while permitting a weiring effect that extends residence time and mechanically enhances the precipitation of solids from liquids. Disadvantageously, this design is limiting in its throughput and necessitates the use of devices for removing solids from the tanks. Conventional baffles force the slurry to follow an extended path needed for the precipitation of untreated solids from solution. Relatively large outlets in the bottom of such tanks are used to remove the settled solids. Disadvantageously, the larger the outlet the more difficult it is to prevent the liquids from entering the solids outlet. Further, to maintain the physical concentration ratios of flocculant to entrained solids when slurry is pre-treated with a flocculating agent, many such systems unintentionally re-circulate water that has already had solids removed, thereby wastefully processing the same volumes of water more than once.
Other prior art better suited to higher throughput of treated slurry is based on sloping bottom tanks of different configurations. For example, U.S. Pat. No. 4,599,117 teaches a ‘v’-shaped tank with fixed tubes rather than baffles. U.S. Pat. No. 5,814,230 teaches a conical tank having a mixer instead of baffles to promote contact between flocculant and slurry. CA 2,756,965 (“'965”) teaches a conical tank having overflow means for allowing lighter material to escape the tank but having no baffles because it relies on water flow counter-current to slurry direction to prevent the buildup of solids on the sloping walls of the conical tank. Disadvantageously, these designs rely on separation occurring within the tank and use baffles in the traditional manner for separation.
CA 2,099,554 to Kallis (“'554”) teaches a transportable cone-shaped tank having a tangential input mid-way between a cylindrical upper and a conical lower section. The fluid outlet of '554 is at the top of the tank using a bowl collector to drain off clean water, but which bowl disadvantageously interferes with access to interior portions of the tank such that no baffles are employed inside the tank. '554 teaches “specially designed and located input and output pipes” and slurry is pumped through a hose into a circular input pipe (called a “centrifuge” pipe) that wraps around the interior of the tank and has slots along its bottom edge to permit solids and fluids to exit into the body of the settling tank. The input pipe of '554 distributes slurry around the tank and extends the time for solids to separate from liquids. By using a tank having a conical base '554 collects solids above a solids outlet having a relatively small area, thereby displacing liquids from direct access to said solids outlet. Disadvantageously the mass of solids so collected passively settles across the conical space coming to rest above the solids outlet. In these circumstances the force compressing the body solids then passing through the plane of the solids outlet is nothing more than the weight resulting from the stationary mass settled into the conical space above the outlet. Consequently, any concentration in these settling tanks is merely incidental to the collection of solids during separation. Although superior to many designs, both '554 and '965 in steady-state operation still allow excess water to reach the centrifuges. Moreover, '554 teaches a valve to block the bottom of the tank in order to permit solids to settle out of solution for periodic removal from the tank. And '965 teaches a jet tube in the conduit connected to the solids outlet to add water to dilute the solids.
Most known means for the conventional processing of drilling slurry are based on settling tanks having baffles (i.e. primary mechanical separators) that act mechanically (like rocks in a stream) to enhance precipitation of solids from fluids (in some cases chemically enhanced). The baffles extend the time that the slurry blend remains inside the tank by extending the length of the path between the inlet and outlet of a settling tank, however the effectiveness of such separation drops as the slurry processing rate increases beyond the capacity of mechanical separators. Consequently, a second stage of separation was appropriate, and when drilling companies were permitted to use local water supplies without restriction, a number of separation system designs solved the problem of insufficient primary separation by adding high-capacity centrifuges in series with the settling tank, requiring substantial electrical power to remove residual water. The high cost of maintaining electrical power and an adequate supply of clean water on a well-site causes the recovery and reuse of drilling water, using minimal power, to be an important objective. Consequently, it is desirable to minimize residual water in the cuttings in order to minimize water reaching the centrifuges (i.e. secondary mechanical separators).